Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Why specify 3ksi concrete for grade beams?

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Hmm. In the hills around here a typical residential foundation is a series of drilled piers connected together by "grade beams". AFAIK nobody considers this to fall under the provisions of  UBC 1921.2.4.1--because exception for "footings" the seems pretty clear. On my drawings I require 3000 psi but state that the design is based on 2500 psi--no special inspection. I observe steel placement, call for max. 3.5" slump, and provide the CBO with a letter at completion.  Most of the time the concrete used is 3/8" pea gravel & a 6 sack mix (also spec'ed on my drawing) because it needs to be pumped.

Aside from the cost of a special inspector & the test coupons, what would you do with a completed foundation of 30 drilled piers & 100 cy of  concrete with a framed structure on it, if the test lab said you only had 2500 psi?
Chuck Utzman, P.E.

THunt(--nospam--at)absconsulting.com wrote:

Dennis,

3000 psi concrete is also required per CBC 2001 section 1921.2.4.1 (and even more restrictive in section 1921A.2.4.1) which is the same as section 21.2.4.1 of ACI 318 2002

Thomas Hunt, S.E.
ABS Consulting




"Dennis Wish" <dennis.wish(--nospam--at)verizon.net>

10/10/2004 10:53 AM


To
<seaint(--nospam--at)seaint.org>
cc

Subject
Why specify 3ksi concrete for grade beams?







You guys had me up most of the night and early this morning as I could not recall why my specifications on my drawings demand f’c=3000 psi concrete for grade beams.
The best I could do was recall when we first established RGA 1-91 back in the late 80’s. At the time, as I recall it, all grade beams used in moment frames were required or strongly suggested to be designed using 3,000 psi concrete because it forced deputy inspection. As I recall it (and I may be wrong but have used this spec since I started doing retrofit work in 1986) the city did not have a structural observation requirement and wanted to force both the engineer of record and a deputy inspector to make sure the concrete exceeded 2,500 psi and that the steel horizontal rebar as well as the shear ties (generally in place if required or not) were adequately placed.
It was a measure to force the community to insure that a mix was adequate.
 
I had one instance where a small frame was to be put in and the contractor decided to mix the concrete himself on site. I objected to this after finding out what he had done. I demanded cores (it was in the city of Beverly Hills) and the stuff came out like soup. The testing lab could not get a sufficient strength without the concrete simply falling apart to find out what strength it had.
 
The contractor complained, but we made him remove all of the concrete he placed and had him do the job over according to the specifications that I provided for special deputy inspection. He paid the price and the work was redone to the satisfaction of the testing lab and the inspector.
 
I think that this practice originated in Los Angeles to force deputy inspection and verification of the strength of the concrete when used to resist bending in the grade beam so as to reduce the column size.
 
I’ve been the engineer of record on hundreds of buildings in the Los Angeles county area from 1986 until 1993 and all of the grade beams I specified were done using f’c=3,000 psi concrete and demanding special deputy inspection. It might have been unwritten, I thought it was stated in RGA 1-91 or in the 1991 UCBC Appendix Chapter 5 but I can not find it so I must assume that it was a recommendation by the professional community at the time.
 
This is not to say that it was required or that every engineer designed to this standard, but it was my practice and since it exceeds the minimum requirements of the code and none of my clients complained, I believe it was the right choice to make.
 
Dennis
 

Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant
dennis.wish(--nospam--at)verizon.net
http://www.structuralist.net
 

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.772 / Virus Database: 519 - Release Date: 10/1/2004